The processing speeds are un-godly these days. A general rule of thumb, the more cores the hotter the computer runs and shorter amount of time the battery lasts. I've always though anything bigger than a 15" defeats the purpose of a laptop...but that's just me.
Here is what I use...
Mid-2010 MacBook Pro:
OSX 10.8/Mint 13
2.4 GHz Intel Core 2
8 GB of RAM
440 GB of storage (120 GB Mercury SSD and 320 GB Seagate)
Comments: Love this computer. High school graduation present, boots both Mountain Lion and Mint 13(64-bit) off the SSD (takes about 21 seconds) while all of the music, photos, movies, and etc. for OSX are kept on the HDD. Runs Office, Matlab, Autodesk Inventor (through Crossover) without issue. Battery lasts for 7-11 hours depending on multiple factors.
Asus X101CH:
Mint 13 (32-bit)
1.6 GHz Intel Atom
1 GB of fixed RAM
320 GB Hitachi HDD
Comments: Recently purchased, enjoying it. Battery lasts about 3 hours. 320 GB drive is way too big for a 10.1" netbook IMO. Planning on upgrading to a 120 GB SSD. Runs Matlab, OpenOffice, CHIRP and Echolink (through Wine) without any issues.
Raspberry Pi:
Raspbian
Comments: Fun little toy. There are some things it does well, and things it doesn't. GUI heavily taxes it but it can still display 1080p.
HP dc5800 SFF:
Ubuntu Server 12.04 LTS (64-bit)
1.8 GHz Intel Pentium Core Duo
1 GB of RAM
2 TB Seagate (no RAID controller but does have another HDD bay)
Asus Bluray Reader
Comments: This is my current server machine (planning to upgrade to a Proliant N40 in the next 2 months). Headless (SSH into it). No GUI. Runs netatalk, makemkv, Handbrake, and Plex. Does get bogged down when ripping using Handbrake. Main use is as a media server and file storage (multipe Windows OS isos, Linux Distros, full program isos...) and I occasionally SSH into the optical drive for the rare instances I need an optical drive on one of my other computers.
One could say I'm a fan of Unix based operating systems. Most of my computers are fairly low in the processing department but they all work just fine for my uses. Don't need the most powerful processor, especially if general computing is primary concern (gaming is where I7's tend to come into play, though a few of my Comp Sci and Electrical friends run I7's on their desktops at home). The I5 is a good blend (I3's are a little slow on Win 7 for my tastes). I would highly recommend a SSD over a HDD if you can find one suitable to your uses. I'd recommend at least 4 GB of memory for any 64 bit Windows 7 on up machine but 2 will work.